1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus configured to acquire a Doppler spectrum image by transmitting and receiving ultrasonic waves to and from a subject and adjust the Doppler velocity range of the Doppler spectrum image, and also relates to a method for generating an ultrasonic image.
2. Description of the Related Art
An ultrasonic imaging apparatus that obtains blood-flow information of a diagnosis site by the ultrasonic Doppler method is known. The ultrasonic Doppler method is a technique of obtaining information on the blood flow in a subject based on the principles of the ultrasonic Doppler method. When ultrasonic waves are transmitted to a diagnosis site having a stream like a blood flow in a subject, the reception frequency slightly shifts from the transmission frequency by the Doppler Effect. This shift frequency (Doppler shift frequency) is proportional to a blood-flow velocity. Blood-flow information is obtained by frequency analysis of the Doppler shift frequency.
The ultrasonic imaging apparatus executes Fast Fourier Transform (FFT) on an obtained Doppler signal. Then, the ultrasonic imaging apparatus displays the spectrum of the result of the frequency analysis, taking frequency f (velocity v) on the vertical axis and time t on the horizontal axis. Measurement of various kinds of items used in diagnosis is executed on the spectrum image.
In a Doppler spectrum image acquired by the pulsed wave Doppler method (PWD), a phenomenon of aliasing (folding) occurs when a sampling frequency fs for the frequency analysis is lower than the Doppler shift frequency. In order to prevent this phenomenon, it is necessary to raise the pulse repetition frequency (PRF), which corresponds to the sampling frequency, and to shorten each interval of observation times. When a position desired to measure is designated, the maximum value of the PRF is necessarily determined. When the PRF is determined, the maximum blood-flow velocity that can be measured is also determined. This maximum blood-flow velocity that can be measured is referred to as the “Doppler velocity range.”
At the time of Doppler spectrum display, a folding portion is generated in a Doppler waveform when the Doppler velocity range is too small. In this case, the operator manually sets a larger Doppler velocity range, with the result that the folding portion falls within a nyquist frequency (half of the PRF), and a smooth continuous Doppler spectrum is obtained in the display. On the contrary, when the Doppler velocity range is too large, the scale of the display is large and the Doppler waveform is displayed small. Such a Doppler waveform displayed small makes observation difficult. In this case, the operator manually sets a smaller Doppler velocity range, with the result that it is possible to display the Doppler spectrum over the full height of a display screen. Besides, it is possible to display the Doppler waveform at a desired position of the Doppler velocity range (vertical axis) by adjusting velocity offset, namely, baseline shift (BLS).
Since the blood-flow velocity varies depending on patients and diagnosis sites, the acquired Doppler waveform also varies depending on patients and diagnosis sites. Therefore, it is necessary to set the Doppler velocity range in accordance with the Doppler waveform. To be specific, the operator needs to adjust the pulse repetition frequency (PRF) corresponding to the Doppler velocity range and the velocity offset (BLS), for each patient and diagnosis site. However, it is cumbersome to adjust the Doppler velocity range (PRF) and the velocity offset (BLS) for each patient and diagnosis site.
Therefore, a technique of automatically determining the Doppler velocity range (PRF) and the velocity offset (BLS) is proposed (for example, Japanese Unexamined Patent Application Publication JP-A 2005-185731).
Conventionally, the Doppler velocity range (PRF) and the velocity offset (BLS) are obtained by assessing past blood-flow conditions and feeding back the result of the assessment. Then, the Doppler velocity range and the velocity offset are tracked in real time by assessing the blood-flow conditions in real time.
However, there is a problem such that a time lag arises because the Doppler velocity range (PRF) and the velocity offset (BLS) are estimated by assessing the past blood-flow conditions. Moreover, the past blood-flow conditions do not necessarily coincide with present and later blood-flow conditions. Therefore, it is difficult to obtain a Doppler velocity range and a velocity offset that are suitable for a Doppler waveform acquired now and later.
Further, conventionally, the operator gives a trigger at a time point when the blood-flow conditions become stable, and after that, the Doppler velocity range (PRF) and the velocity offset (BLS) are estimated. However, when a variation such as breathing of a subject and displacement of an ultrasonic probe occurs after the operator gives the trigger, it is impossible to obtain the Doppler velocity range and velocity offset suitable for the Doppler waveform, and it is necessary to execute the estimation process again.